Best Practices from the 501G fleet: Athens and Granite Ridge

Redesign of Athens’ electric system improves operating flexibility, emergency response

Athens Generating and Granite Ridge Energy were the only two plants in the Siemens 501G fleet recognized for their best practices in the 2015 program sponsored by CCJ. Both facilities are operated by NAES Corp. The former has three 360-MW power blocks, each equipped with a 501G gas turbine, HRSG, and Siemens HE steam turbine. Plant Manager Dan DeVinney says the power blocks can be dispatched independently of each other. Granite Ridge is a 752-MW combined cycle with two power blocks managed by Bill Vogel.

Athens was constructed with emergency systems, of course, but they had limitations regarding emergency response and maintenance, as plant personnel came to learn with experience (photo). The following shortcomings were identified with the facility’s electric distribution system:

      • Inability to cross-connect unit power distribution during emergency and maintenance situations.

      • Emergency diesel/generator (EDG) was not loaded to its design capability.

      • Battery charger was undersized and battery installation was inadequate.

      • Lack of power feed from the essential service bus to critical plant loads.

      • Single source of power to critical maintenance equipment.

The original design (black line-work in diagram) impacted both emergency response and maintenance activities. For example, if a unit lost power, the gas-turbine (GT) batteries had to carry all critical loads—including GT dc lube oil, turning gear, and dc air-side seal oil—without any backup source of power. If power was not restored in approximately eight hours, the GT would lose all seal oil and lube oil.

Unit maintenance outages also were affected by the original electrical line-up: Without the ability to cross-connect 4160-Vac busses among units, a block had no power once it was disconnected from the switchyard. Hence, all heat-trace, welding outlets, and crane service were de-energized. This made winter maintenance outages very challenging.

First step in the solution: A plan was developed to cross-connect power among units (red line-work at bottom of diagram), enabling a block to receive power from a neighboring unit’s 4160-Vac bus while it was disconnected from the switchyard backfeed. This required (1) a distribution study; (2) installation of interlocking relays, potential transformers, current transformers, and DCS logic; and (3) operator training.

The lack of power feed from the plant’s essential service bus to critical plant loads was addressed next. The existing battery charger was supplied via the GT ac motor control center (MCC) or, on loss of ac power, from the batteries. This electrical configuration adversely impacted emergency recovery. Restoration was particularly challenging when the fault was offsite and power could not be restored promptly.

Existing spare service was identified on the unit essential bus. Service then was pulled from the unit essential MCC to the GT battery charger and a single ac seal-oil pump. The existing GT valve regulated lead-acid batteries (VREL) then were replaced with flooded lead-acid batteries. In addition, the original battery charger was upgraded from 175 to 300 amp. These upgrades ensured that the GT had a reliable source of dc power during all plant conditions.

Finally, the single source of power to plant maintenance equipment was addressed. Power originally sourced from a single unit now is sourced from two.

Athens fig 1

Key Athens personnel involved in the redesign of the plant’s electric distribution system included Plant Engineer Hank Tripp and EI&C Technicians Rob O’Connell and Bob Robinson (l to r). Also involved, but camera shy, were EI&C Technicians Todd Wolford and Eric VanZandt

Key Athens personnel involved in the redesign of the plant’s electric distribution system included Plant Engineer Hank Tripp and EI&C Technicians Rob O’Connell and Bob Robinson (l to r). Also involved, but camera shy, were EI&C Technicians Todd Wolford and Eric VanZandt

Very positive results were realized in the plant’s maintenance activities and emergency response. First, the plant conducted three GT major inspections from 2011 to 2013 and replaced the control systems. Two of the majors and the control-system upgrades were done during the winter months. Using the 4160-Vac cross-tie, power was maintained among neighboring units. This kept all lighting, crane, welding and heat-trace services available during these complex maintenance evolutions. Outages were completed on schedule with no major deficiencies.

Second, the plant has experienced several unit power disruptions since the modifications were completed. In all cases, the EDG starter was connected to the unit essential MCC and began supplying power to GT and steam turbine critical loads. During emergencies, the EDG now is loaded closer to its design point. Once the cause of a disruption is identified, the unit is placed on the cross-tie feed, eliminating the need for emergency pumps, dc systems, and EDG. This flexibility significantly decreased the duration of forced outages. In sum, the units are more flexible and more reliable because of the electrical enhancements.

Arc-flash mitigation measures at GRE greatly reduce hazard risk to staff, equipment

Granite Ridge Energy (GRE) conducted an arc-fault assessment in 2010 as part of NFPA 70E compliance to determine the arc-flash potential of site electrical equipment and to assign Hazard Risk Categories (HRC) in accordance with NFPA recommendations. Although changes in protective device settings were made in an attempt to reduce all HRCs to Level 2 and below, six main electrical buses were identified at a Level 5 for which there is no safe arc-flash protection; many devices remained at HRC 3 and 4 which had the potential for severe arc-flash events.

This possibility posed a serious risk to employees working on or near the equipment, requiring the use of full flame-resistant (FR) suits and hoods. In addition, an arc-flash event could cause severe damage to electrical equipment which ultimately could affect plant reliability and availability.

Risk mitigation approach. Maintenance Manager Dan Jorgensen and other plant personnel worked with power systems specialists at Three-C Electrical to implement arc-flash mitigation measures in three phases. The project took four years to complete because of the need to coordinate work with full plant outages.

Phase One was completed in 2010, in conjunction with the initial evaluation. It focused on changes in the relay systems settings to improve trip response time.

Phase Two, conducted in 2013, involved installation of an optical relay system in the three main MCC (motor control center) rooms. The ABB system, called REA, is designed to give split-second trip commands to all circuit breakers that may feed an arc fault in low- or medium-voltage air-insulated metal-clad switchgear. The system uses a non-shielded bare-fiber sensor that detects light along its entire length. Fiber loops were run through each of the 480-V main switchboards.

On detection of an arc fault, the REA delivers trip commands in less than 2.5 milliseconds to all circuit breakers feeding the fault zone. In addition to arcing short circuits, arcing earth faults with current levels below the normal load current can be detected and interrupted. In the event of a fault, indicator lights on the REA panel guide personnel to the exact fault zone for troubleshooting and corrective maintenance. The system protects the feeder breakers, main switchboards, and the substation transformers. Although this system greatly reduced the arc-flash potential, it was determined that many feeder breakers for the MCC rooms had antiquated trip kits and were thus missing instantaneous protection.

Phase Three, the final step of the project, was completed in 2014 with the installation of secondary digital trip kits on eleven 480-V feeder breakers to allow for instantaneous overcurrent protection on the MCCs fed from the main switchboards.

Safety was improved dramatically by installation of the multi-phase arc-fault protection system: Arc-flash level on the majority of HRC 3-, 4-, and 5-rated electrical devices requiring live interaction were effectively reduced to HRC 2 or below. Operating procedures were developed for the remaining devices to de-energize upstream and eliminate the need for live work. This greatly reduced the hazard risk to employees and equipment in the event of an arc fault. As a result of the lower HRC, Granite Ridge was able to standardize to a single clothing protection system which greatly increased the safety to our employees.

RO addition at GRE reduces sulfate discharges, boosts demin water production

Granite Ridge Energy (GRE) discharges wastewater to the sewer system under a local discharge permit which includes a pounds-per-day limit for sulfate. The plant’s wastewater stream includes cooling-tower blowdown, sand-filter backwash water from the grey-water pretreatment system, and demineralizer-resin regeneration water. Note that the plant uses treated effluent (grey water) from the city wastewater treatment plant as cooling-tower makeup.

Challenge. High sulfate levels in plant wastewater originating primarily from sulfuric acid use in demin resin regeneration were reaching permit limits and causing GRE to take remediation measures that negatively impacted operating costs. A reduction in sulfate loading of at least 20% was needed to alleviate the problem. Factors affecting the sulfate loading evaluated during the assessment phase of the project included the following:

      • Increased conductivity of potable water used to produce demin caused shorter production runs and increased regenerations of the demineralizer resin, which uses sulfuric acid.

      • Poor-quality grey water required excessive pre-treatment filter backwashes which increased wastewater discharges, thereby increasing mass sulfate output.

      • Sulfuric acid used in cooling-tower pH control is discharged in the blowdown.

In addition to increased sulfate, the plant also was facing challenges (1) maintaining sufficient cooling-water production through the grey-water pre-treatment filtration system, and (2) providing sufficient potable water to meet demin-water production requirements.

Solution. Operations Manager Jim Barrett and GRE staff looked to address all three challenges with one solution: The installation of a single-train, 80-gpm reverse-osmosis (RO) system ahead of the existing demin system. It provided a 50% increase in demin water production. Redundancy was not needed at GRE because the demin water tank holds a nominal seven-day supply and the demineralizers can be operated without the RO system

To ensure optimal performance, GRE opted for a modular approach, keeping the RO system segregated from the existing demin system by installing a 1500-gal storage tank between them. The modular approach allowed engineers to locate subsets of the RO system in different locations within the water-treatment building for optimal use of floor space.

The RO system sends permeate to the storage tank as a standalone operation in which the tank level controls RO on/off. A permeate booster pump supplies water from the storage tank at the pressure needed to operate the demineralizers. Permeate flow is regulated with a manual bypass valve. Operationally, the systems are balanced to provide continuous operation of the RO by matching its output to demineralizer demand.

High-quality water produced by the RO system is polished in the existing cation/anion demineralizers. The reduction in total dissolved solids and conductivity achieved by the RO system allows the demineralizers to produce 50% more high-purity water than previously. An increase in demin production is achieved because the existing demineralizers can now be operated in parallel, reducing system pressure drop and increasing flow. RO permeate is 98% cleaner than the potable water available to GRE, extending demineralizer throughput 16-fold and increasing the time between resin regeneration cycles from hours to days.

Results. Approaching the sulfate-reduction challenge with other plant improvements in mind allowed GRE to resolve sulfate permit-limit concerns and increase both cooling-water supply and demin production. Additionally, GRE entered into a long-term lease agreement for the RO system which includes scheduled maintenance and remote monitoring to support optimum RO efficiency 24/7, while reducing overall budget dollars. The RO system has reduced GRE’s monthly demin water cost by $2000 to $12,000, depending on demand. Year-round monthly average saving is $6300.

In sum, RO system benefits include the following:

      • Reduction in sulfate discharges by 25%.

      • Increase in demineralizer throughput between regenerations of 40,000 to about 650,000 gallons.

      • Reduction in demineralizer regeneration chemical costs of 93%.

      • Use of RO concentrate/reject water as cooling-tower makeup increased the amount of cooling water available to GRE by 30,000 gal/day.

      • Increased in daily demin water production capability from 80,000 to 120,000 gal/day.

Posted in 501 F&G Users Group |

Comments are closed.